1. Field of the Invention
The present invention relates to an induction-type motion sensor
2. Description of the Prior Art
Sensors for detecting motion which operate on the principle of relative movement between a conductor and a magnetic field resulting in a current being induced in the conductor are known in the art. Such induction-type sensors generally include a magnetic circuit with a permanent magnet, the magnetic circuit comprising a stator, a rotor and an electrical coil devised around the stator. The rotor is comprising a pendulum arranged to oscillate when the motion sensor is subjected to movement, and a moving element is provided at the stator, at least partially in the stator's air gap. The moving element is connected to the pendulum and, when the element is set in motion by the pendulum, the element acts on the magnetic field in the stator, thereby inducing a current in the electrical coil.
Brazilian Patent No. 8 501 719 describes an inductive power generator with a bipolar permanent magnet as the rotor element, which is rotatable in the air gap of the stator. An electrical coil is provided around the stator. A pendulum oscillates when subjected to movement stress and transfers the movement to the rotor element via a system of gears. The gears cause an upward shift in the number of oscillations of the rotor element in the air gap, compared to the pendulum oscillation, so as to achieve a greater number of alternations in the magnetic field of the stator when the pendulum oscillates. The current generated in the electrical coil in this way is rectified and used for powering a watch.
The induced current changes direction when the magnetic field alternates. A low gearing transmission ratio produces a smaller number of alternations in the magnetic field for a given pendulum travel, whereas a high gearing transmission ratio produces a larger number of alternations in the magnetic field with the same pendulum travel. However, higher gearing transmission ratio also means that heavier movement stress is required to produce the same pendulum travel, i.e., the higher the gearing transmission ratio, the larger the movement force needed to induce the oscillate. In addition, a design employing gears is complex and space-consuming. With a plurality of gears, total friction is greater in the sub-elements of the rotor, and the pendulum, for this reason, is more difficult to set in motion.
An inductive power generator having a magnetic circuit with a permanent magnet, is described in PCT application WO 84/01041. The magnetic circuit includes a stator with an air gap, an electrical coil,and a rotor in the form of a semicircular pendulum disc with a plurality of permanent magnets located so they pass through the air gap when the pendulum oscillates. Increasing the number of magnets on the pendulum disc increases the number of alternations in the magnetic field and accordingly results in a higher frequency for the induced current.
The power generator is designed to generate as much energy as possible from a given movement force and, in particular, to generate energy from large movement forces, since a small swing of the pendulum only produces a limited change in the magnetic field. The version with a plurality of magnets on the pendulum disc means that the magnets must either be made smaller or the pendulum disc must be made larger. Reducing the size of the magnets also reduces their magnetic field strength, thereby reducing the induced current. Moreover, since the pendulum disc only consists of a semicircle, no magnets will be in the air gap for half the rotation cycle when the pendulum oscillates around its oscillation axis.
The above-described generators are designed to transform kinetic energy into electrical energy in an efficient way, but they can also serve as motion sensors, since they emit a specific electrical signal for each movement signal. However, it is important for a motion sensor, intended for use e.g., in an implantable medical apparatus in order to detect body movements, to display high sensitivity to small movements and changes in movement. For example, the motion sensor must be able to emit a signal which differentiates between a patient's normal walking and rapid walking. Sensitivity in an induction-type motion sensor depends on the sensor's ability, on the basis of movement force, to generate an identifiable signal which can be used as a parameter related to movement stress. Alternations in the direction of the induced current comprise one appropriate parameter. The generator according to Brazilian Patent No. 8 501 719 produces a large number of alternations in current direction through high gearing in the system of gears. As noted above, however, high gearing simultaneously demands greater movement force to set the pendulum in motion. Sensitivity to movements is therefore limited to the gearing selected. This can be circumvented with a generator according to PCT application WO 84/01041 in which the number of magnets on the rotor disc produces a large number of alternations in current direction without any system of gears. However, a magnet can become too small to still retain a field strength sufficient to induce a measurable current. Further, more magnets make the peripheral part of the pendulum disc heavier, thereby affecting the disc's natural frequency and the movement force required to set the pendulum in motion, since the inertia of the pendulum disc accordingly increases.
The difficulties in using the known power generators as motion sensors become more pronounced if the sensor is intended for use in an implantable medical apparatus. Space in these apparatuses is limited, and developments are tending to reduce this space even further. Both the system of gears and the pendulum disc with a plurality of magnets are too space-consuming for such applications, and are also too insensitive to small movements if used as motion sensors.